The present invention relates to fiber optic communications systems, and more specifically, to amplification of optical signals propagating in an optical fiber.
Optical signals for conveying information in a fiber optic communication system experience attenuation as the optical signals are transmitted though an optical fiber over extended distances. The attenuated optical signal can be regenerated using amplifiers such as optical fiber Raman amplifiers, which rely on stimulated Raman scattering to transfer energy to the optical signal. The optical fiber Raman amplifier comprises a fiber that receives two input beams: a pump beam and the optical signal. Energy in the pump beam is coupled into the signal beam through stimulated Raman scattering, and the optical signal is thereby amplified upon passing through the fiber amplifier. The extent of amplification or gain depends on the relation between the polarization of the pump beam and that of the signal beam. If both the pump beam and the signal beam are linearly polarized and have electric fields oriented in the same direction, then the gain is higher than if the electric fields are oriented perpendicular to each other. Disadvantageously, fluctuations in the polarization of the signal or pump beam that cause the relative orientations of the electric fields to vary produce fluctuations in the gain of the amplifier. For example, the gain will decrease for pump and signal beams that initially have electric fields oriented parallel but are reoriented so as to no longer be parallel. Conversely, gain will increase if the beams are initially perpendicular but subsequently contain parallel components. Such fluctuations in the gain cause variations in the intensity of the optical signal, which introduces noise into the signal and thereby increases the likelihood of errors in transmitting information over optical fibers.
In conventional systems designed to minimize fluctuations in gain, the pump beam is provided by two or more semiconductor lasers that output polarized light. The polarized light is directed to a coupler that combines the light from the different semiconductor lasers after first separating the respective beams into perpendicular polarizations. For example, in the case where two semiconductors are employed to pump the fiber amplifier, light emitted from the two semiconductors is input into the coupler. The coupler causes the polarized light beams from the two semiconductor lasers to have electric fields oriented perpendicular to each other and produces a combined beam that is then directed to the optical fiber Raman amplifier.
Although employing a plurality of semiconductor lasers can reduce the fluctuations in gain, requiring more than one semiconductor laser adds to the complexity of the amplifier. What is needed is a design for an optical fiber Raman amplifier that is simpler and less expensive yet that minimizes the fluctuation in gain caused by variations in polarization of the pump and signal beams.